1. Field of the Invention
The present invention relates to a laser welding method of welding a plurality of flat works, which are overlapped with each other, having coating on the surfaces with a laser beam. The present invention also relates to a laser welding device to which the laser welding method is applied.
2. Related Art
In general, in the case where flat works such as galvanized steel sheets, on the surfaces of which coating is provided, are overlapped with each other and welded by a laser beam, since a boiling point of zinc provided on the steel sheet surfaces is lower than a melting point of the sheet material, zinc is vaporized during the process of welding and zinc vapor is generated and the thus generated zinc vapor remains in the welding portion and further the thus generated zinc vapor blows out the melted steel sheet material, which could be a cause of a defect of welding such as blowholes or pits. In order to prevent the occurrence of the above problems, it is necessary to properly discharge the vapor of the zinc coating from the steel sheet surface to the outside.
Therefore, the following methods are conventionally known. Disclosed in JP-A-2002-263871 is a method in which one of the works is irradiated and heated with a laser beam so that the works can be separated from each other, and another laser beam is irradiated to the separated portion. However, this laser welding method, in which two lasers are used, is disadvantageous in that the manufacturing cost is raised because one of the two lasers is used as a heat source for heating the work. In order to solve this problem, disclosed in JP-A-2003-025082 is a method, as shown in FIG. 7 of the present description, in which galvanized steel sheets 101, 102 are overlapped and welded by a laser beam. A portion, in which the galvanized steel sheets 101, 102 are overlapped, is preheated by a plasma heat source 107, and then the primary laser beam welding is conducted by a laser beam 108.
According to this laser welding method, a portion of the upper side of the upper side galvanized steel sheet 101 is melted by a plasma heat source 107 for preheating which precedes in the welding direction 121, and heat is transferred to a region, which is not melted yet, located in a remaining region in which the steel sheets are overlapped. In this way, zinc plating 104, 105 provided on the face, on which the steel sheets are overlapped, is heated and vaporized. At this time, the upper side galvanized steel sheet 101 is deformed by the heat generated by the plasma heat source 107. Accordingly, a gap is formed between the galvanized steel sheets 101, 102. Next, at a position distant from the heating position of the plasma heat source 107 by a predetermined distance L, the upper side galvanized steel sheet 101 and the lower side galvanized steel sheet 102 are heated with a laser beam 108 for welding. Due to the foregoing, while galvanizing vapor is being discharged from the gap, the galvanized steel sheets 101, 102 are welded being penetrated.
However, in this laser welding method, since the plasma heat source 107 is used, in order to form a gap between the galvanized steel sheets 101, 102 and in order to heat and vaporize the zinc plating 104, 105 on the surface, on which the galvanized steel sheets 101, 102 are overlapped, it is necessary that an upper face of the upper side galvanized steel sheet 101 is widely melted exceeding the irradiation beam width of the laser beam 108. Therefore, the corrosion prevention property and the appearance of the surface of the galvanized steel sheet 101 are deteriorated, that is, the surface property of the steel sheet is damaged. Further, since the plasma heat source 107 is used, in order to stabilize a heat input, it is necessary to increase an output. Therefore, an excessively large quantity of heat is given to the galvanized steel sheet 101. Accordingly, a heat deformation of the galvanized steel sheet 101 is increased.
The present invention has been accomplished to solve the above problems. An object of the present invention is to provide a laser welding method capable of easily adjusting a quantity of heat to be inputted and also capable of substantially eliminating a heat deformation of a work and a damage caused on a work surface. Another object of the present invention is to provide a laser welding device to which the laser welding method is applied.